Various types of recognition systems are taught by the following commonly assigned U.S. Patents and Applications: U.S. Pat. Nos. 5,287,112; 5,270,717; 5,196,735; 5,170,493; 5,168,282; 5,126,745; 5,073,781; 5,053,774; 5,025,492; U.S. Ser. No. 08/021,123, filed Feb. 23, 1993 TI-17529!; Ser. No. 08/065,286, filed May 21, 1993 TI-16981!; and Ser. No. 08/086,786, filed Jul. 2, 1993 TI-17507!. Systems conforming to the teachings of the foregoing documents are marketed under the name TIRIS ("Texas Instruments Register and Identification System"). A recognition system similar in result to, but structurally and functionally specifically different from, TIRIS is disclosed in U.S. Pat. No. 4,918,955. Other types of recognition systems include systems known as AVI (for "Automatic Vehicular Identification"), as taught, for example, by commonly assigned U.S. Pat. No. 5,287,112 and commonly assigned U.S. application Ser. No. 08/021,123, filed Feb. 23, 1993.
In one type of TIRIS recognition system of interest, a transmitter/receiver (also referred to herein, in the claims hereof and elsewhere as an "interrogator" or a "reader") selectively radiates energy via an associated first antenna. The energy is radiated from the vicinity of a key-operated lock, such as a vehicle ignition switch. The radiated energy is often referred to as an "interrogation signal." The radiated energy is received by a second antenna present on or in a key. The key also includes facilities, such as circuitry (sometimes called a "transponder" or "tag"), connected to the second antenna. An electrical signal produced or induced in the key's circuitry by the received energy either is affected (e.g., increased or decreased) by the circuitry or effects the generation of a stored, uniquely coded signal by the circuitry. The affected or coded signal is often referred to as a "recognition signal."
Depending on the constituents of the key's circuitry, either the recognition signal is transmitted or reflected back to the transmitter/receiver via the second and first inductors or antennas. A key "matching" the particular ignition switch transmits or causes a predetermined recognition signal. Other keys which operate other ignition switches associated with similar recognition systems may similarly respond to the interrogation signal but transmit recognition signals different from the predetermined recognition signal.
The transmitter/receiver includes facilities which analyze the recognition signals received thereat to determine if the analyzed signal is the predetermined recognition signal produced by the matching key. If the analyzed signal is the predetermined recognition signal, the concurrence of such predetermined recognition signal and the operation of the ignition switch by the matching key starts the engine of the vehicle. If the analyzed signal is not the predetermined recognition signal, either the ignition switch cannot be operated by the key, or, if it can be operated, such operation is ineffective to start the engine.
Portability and/or space limitations usually result in the transmitter/receiver of a TIRIS-type of recognition system being not very powerful. Also, the recognition signals, that is the signals transmitted or reflected back to the transmitter/receiver from the key-included circuitry, may be derived from the limited energy radiated from the transmitter/receiver, not from energy derived from a key-contained power source, such as a battery, as is typical in systems of the AVI type. While the use of a battery with key-included circuitry of a TIRIS system is technically possible, the large size and resulting unwieldiness of the resulting key would probably lead to rejection by users. As a consequence of the foregoing, it is critical that circuit efficiencies be as high as possible.
One type of recognition circuitry of the subject type includes active and passive components, which in response to the receipt of energy from the transmitter/receiver produce a coded signal. See the above-noted commonly assigned U.S. Patents and Applications. The coded signal, which may be produced by data stored in memory, is transmitted back to the transmitter/receiver, where comparison with the stored "matching" signal is carried out. The coded signal may be produced by modulating a carrier with the stored code, and the carrier may be, or may be derived from, the energy received by the key-included facilities from the transmitter/receiver. In this latter event, the system may be of the TIRIS variety, and the key and its circuitry require no on-board power source and may be said to be "batteryless." This third type of system may also be of the AVI variety, in which case the transponder is typically powered by a self-contained power source.
Recognition systems of the above type may be the full-duplex variety. Specifically, the transmitter/receiver may simultaneously operate as both a transmitter and a receiver, that is, it may simultaneously radiate energy to the key-included circuitry and receive for analysis the corresponding signal produced by such circuitry. Typically, in full-duplex operation, the frequency of the modulated carrier radiated by the transmitter/receiver to the key-included circuitry is different from the frequency of the modulated carrier produced by the key-included circuitry and thereafter received and analyzed by the receiver/transmitter. See above-noted commonly assigned application Ser. No. 08/012,123 TI-17529!.
Available full-duplex transponders contain full-wave rectifiers that provide the circuits on a single silicon chip with a DC voltage. This voltage, which is usually in the order of 2 volts, is on the order of 0.7 to 1.5 volts lower than the maximum alternating current peak voltage at the input of the rectifier. This voltage drop has its problems in long-range operations, because the available alternating current voltage, by the time it reaches its destination, is much lower than desirable.
Conventional rectifiers circuits for full-duplex transponders include circuitry that transmits radiofrequency energy from a reader unit to the transponder via the electromagnetic coupling of the two antennas. In such a circuit, the sinewave alternating current voltage between the two coils is reflected in order to generate, using a full-wave rectifier circuit, a direct current voltage V.sub.DD. The conventional rectifier circuits include four diodes that are associated to rectify the alternating current voltage. When the diodes are conducting, the coils connecting to the electromagnetic coupling are negative, typically having a value of approximately -0.6 volts. Also, the maximum value of V.sub.DD is smaller than the maximize voltage at the coils. As a result, it is expensive to realize diodes in an implementation of a rectifier circuit wherein the diodes are independent from the substrate, such as with a Schottky diode or n-poly/p-poly diode. In essence, there is the need for a more efficient full-duplex transponder circuit that includes a full-wave rectifier.
One attempt to include the rectifier circuit includes a network of simple diodes associated with N-channel transistors. In such a circuit, the negative halfwave of the alternating current is fed through a circuit. In order to avoid current flow in reverse direction, a P-channel transistors works as a diode. The problem with working with the P-channel transistor as a diode is that it is not possible to use this circuit with the circuit that employs EEPROM capacitive trimming. For transponder operation, this limitation is not acceptable.
In addressing the particular needs of a transponder circuit operation, four N-channel transistors have been used to form a rectifier circuit. In this circuit, and due to the active switches N1 and N2, there are nearly no negative voltage drops. There are no other diodes necessary. In addition, capacitive trimming is possible for large N-channel transistors. A disadvantage with this circuit, however, is that the voltage drop across the N-channel transistors that supply VDD can be excessive, for example, on the order of 1.6 volts. Such a circuit is described in K. Klosa, Kontaktlose RF-indentifikationssyteme-Funktionsweise, SCHALTUNGSTECHNIK UND REALISIERUNG DES RF-INTERFACES, GME Mikroelectronik, Dresden, Germany, 1993.